Micro-machined sensors and actuators have been investigated and produced industrially for about 20 years. A range of micro-mechanical sensors has been developed for applications such as process control, bio-medical applications, consumer products and automobiles. Such sensors and actuators may be based on capacitive and piezo-electric sensing schemes, and the piezo-resistive effect (ie stress-induced change in resistivity of the sensing material) has also been widely used.
Until about ten years ago, most piezo-resistive sensors were fabricated on single-crystal silicon (c-Si) substrates using either diffusion or implantation doping techniques to form the piezo-resistors on bulk micro-machined mechanical sensing structures. However, this approach has its limitations. For example, the reverse-biased pn junction that isolates a doped resistor from the substrate can fail at high ambient temperature, which seriously limits the range of possible applications for such devices. While such problems can be overcome by using silicon-on-insulator or silicon-on-sapphire substrates, this is at the expense of substantially higher production costs. Preferred to c-Si therefore is the dielectrically isolated polysilicon and a poly-Si piezoresistor is a more viable alternative.